This invention relates to magnetic recording media, and more particularly, to magnetic recording media of metal thin film type, and a method for conducting recording/reproducing operation in such media.
Among magnetic recording media for use in video, audio and other applications, active research and development works have been made on magnetic recording media, usually magnetic tapes having a magnetic layer in the form of a continuous thin film because of the compactness of a roll of tape.
The preferred magnetic layers for such continuous metal film type media are deposited films of Co, Co-Ni, Co-O, Co-Ni-O and similar systems formed by the so-called oblique incidence evaporation process in which cobalt and optional elements are evaporated in vacuum and directed at a given angle with respect to the normal to the substrate because such evaporated films exhibit superior characteristics.
These magnetic recording media, particularly for use as magnetic tape and magnetic discs, must fulfil a number of requirements including low dynamic coefficient of friction, smooth and stable travel performance for a prolonged period, improved wear resistance, stability under storage environment to ensure consistent reproduction, and durability (durability of tape both during normal operation and in the still mode).
A variety of pre-treatments have heretofore been made on various base films or substrates for the purpose of improving durability. Such pre-treatments include treatments with chemical solution, coating, corona discharge treatment, and the like.
Chemical treatments may be treatments with acid and alkali. Among such chemical treatments most effective is by oxidizing the surface of a base film with a chemical solution of a strong acid and/or a strong oxidizing agent, for example, chromate solution, and introducing carbonyl or carboxyl radicals to etch the surface. The chemical treatments, however, require subsequent rinsing and drying of film surface and a great investment is needed for the treatment of spent liquid. Particularly, chromate treatment yields a spent liquid which must be severely treated for environmental pollution control, and its commercial utilization is now diminishing.
The film coating technique is by coating a base film with an undercoat on which a magnetic film is formed. The interaction between a binder in the undercoat and the magnetic film is necessary. The composition of the undercoat must be selected to meet a particular binder and pigment used in the magnetic layer. The coating techniques reguire not only such a careful choice, but also coating and drying steps. Of course, the consumption of coating material leads to the increased cost of products.
The corona discharge treatment is advantageous because of dry nature eliminating the need for additional steps of rinsing, drying, and disposal of spent liquid. Corona treatment has been carried out for many years and is effective in improving adhesion, wettability, and printability. The corona treatment, however, is not successful in improving the properties of magnetic recording media to such an extent as to fulfill the high performance which is imposed on the present day and future magnetic recording media.
Another technique known in the art is a flame treatment which is difficult to apply to magnetic recording media which reguire a high degree of dimensional stability.
Under these circumstances, a proposal is made to treat base films or substrates with a plasma. The plasma treatment is a one-step dry process and thus has the advantage that drying and disposal of spent solution are unnecessary and no extra material like binders is consumed. In addition, the plasma treatment enables high speed, continuous production so that it can be readily incorporated in the process of manufacturing magnetic recording media without sacrifying production speed and yield.
One technique for plasma treatment of substrates is disclosed in Japanese Patent Publication No. 57-42889 (published on Sept. 11, 1982) wherein a treatment is effected with a plasma having a frequency in the range of radio frequency to microwave using a treating gas of air, oxygen, nitrogen, hydrogen, helium, argon, etc. The radio frequency of 13.56 MHz is only described in this publication.
Also, Japanese Patent Application Kokai No. 58-77030 (laid open on May 10, 1983) descloses a process of plasma treatment by applying an AC current at the commercial frequency between electrodes using a treating gas of oxygen, argon, helium, neon or nitrogen. These plasma treatments are somewhat successful in improving the adhesion of a treated base film to a magnetic layer and hence, the durability of magnetic recording media, but not fully satisfactory in bond strength and durability.
These media should have a flat surface because of remarkable deterioration of their properties due to a spacing loss. However, as the surface becomes flatter, the friction becomes greater adversely affecting head contact and transport movement.
Usually, the metal thin film type media have a magnetic layer as thin as 0.05 to 0.5 .mu.m so that the surface property of the media depends on the surface property of the substrate. For example, Japanese Patent Application Kokai No. 53-116115 discloses the provision of gently sloping protrusions in the form of creases or wrinkles on the substrate surface. Also, Japanese Patent Application Kokai Nos. 58-68227 and 58-100221 disclose the location of fine particles on the substrate surface, resulting in surface irregularities observable under an optical microscope with a magnifying power of 50 to 400 and actually measureable for height by means of a probe surface roughness meter (height 100 to 2000 .ANG.). These media are improved to a more or less extent in physical properties such as dynamic friction, runnability (the durability of tape which travels in firctional contact with rigid members in a recording machine), and moving stability as well as in electromagnetic properties.
Various physical and electromagnetic properties of the ferromagnetic metal thin film layer can be further improved when the magnetic layer contains oxygen, and particularly when an oxide coating of ferromagnetic metal (Co and/or Ni) is formed at the magnetic layer surface, for example, by carrying out the formation of the ferromagnetic thin film layer in the presence of oxygen under a predetermined partial pressure. The above-mentioned gently sloping protrusions in the form of creases or wrinkles are less effective particularly when the oxide coating is formed at the magnetic layer surface.
In Japanese Patent Application Kokai No. 58-68227, fine protrusions are distributed at a density of at most about 10.sup.6 per square millimeter. Video tape recorders utilize the minimum recording wavelength of less than 1 .mu.m, for example, about 0.7 .mu.m. The magnetic layer having an oxide coating on its surface provides insufficient physical and electromagnetic properties at such a recording wavelength.
Japanese Patent Application Kokai No. 58-100221 describes Examples 1 and 2 where fine protrusions having a height of 300 to 500 .ANG. are distributed at a density of about 10.sup.4 to 10.sup.6 per square millimeter. These magnetic layers are regarded to be free of oxide at the surface and thus exhibit different travel durability behavior than magnetic layers having an oxide coating at the surface. The presence of an oxide layer on the magnetic layer surface requires that fine protrusions be distributed in optimum correlated size and density. Differently stated, prior art metal thin film type magnetic media are still not satisfactorily improved in durability, bond strength, and physical and electromagnetic properties.